1. Field of the Invention
The present invention relates to generally to curable coating compositions and more particularly to a curable polymethylsilsesquioxane composition that provides a highly water-repellent and strongly corrosion-resistant cured film.
2. Description of the Prior Art
Silicone resins that contain 1.5 oxygen atoms per silicon atom are generically known as polysilsesquioxanes. The polysilsesquioxanes are highly heat resistant and exhibit good electrical insulation properties and flame retardancy. Because of these properties, polysilsesquioxanes have been used as resist materials and interlayer dielectric films in semiconductor fabrication (see, among others, "Shirikoon Handobukku" (English title: Silicone Handbook), edited by Kunio Itoh, published by Nikkan Kogyo Shinbunsha (1990)).
Methods are already known for the synthesis of polymethylsilsesquioxane. For example, polymethylsilsesquioxane can be synthesized by dissolving methyltrichlorosilane in the presence of amine in a single solvent or mixture of solvents selected from ketones and ethers, adding water to this system dropwise to effect hydrolysis, and then heating to effect condensation (see Japanese Patent Publication (Kokoku) Numbers Sho 60-17214 (17,214/1985) and Hei 1-43773 (43,773/1989) and U.S. Pat. No. 4,399,266). Another synthesis example is taught in EP 0 406 911 A1. This reference teaches the dissolution of a trifunctional methylsilane in organic solvent;
then hydrolysis by the dropwise addition of water to this solution at a temperature from -20.degree. C. to -50.degree. C. under an inert gas pressure of 1,000 to 3,000 Pa; and thereafter condensation by heating. Yet another synthesis example is disclosed in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3- 20331 (20,331/1991). This reference teaches the reaction in organic solvent of methyltriacetoxysilane with an equivalent amount of alcohol and/or water to synthesize the alkoxyacetoxysilane; polycondensation of the alkoxyacetoxysilane in organic solvent in the presence of sodium bicarbonate to give a prepolymer; and condensation of this prepolymer by heating in the presence of at least 1 catalyst selected from the alkali metal hydroxides, alkaline-earth metal hydroxides, alkali metal fluorides, alkaline-earth metal fluorides, and triethylamine. Yet another synthesis example is found in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3-227321 (227,321/1991). This reference teaches the dissolution of alkali metal carboxylate and lower alcohol in a mixed liquid system that forms two phases (water and hydrocarbon solvent); the dropwise addition of methyltrihalosilane into this system to effect hydrolysis; and condensation by heating.
The polymethylsilsesquioxanes afforded by these methods share a characteristic feature: they are hard but brittle. Some of the preceding references even include tactics for addressing this problem. Japanese Patent Publication (Kokoku) Number Hei 1-43773 instructs regulating the fraction with molecular weight.ltoreq.20,000 (molecular weight as determined by gel permeation chromatography (GPC) calibrated with polystyrene standards) to 15 to 30 weight % of the polymethylsilsesquioxane. However, even this does no more than enable the preparation of coatings with thicknesses of about 1.8 to 2.0 .mu.m. Similarly, the technology in EP 0 406 911 A1 can only provide films with maximum thicknesses of 3 to 3.5 .mu.m without cracking. At larger film thicknesses cracking occurs, and of course the flexibility that would permit the fabrication of an independent film is absent.
We have already discovered (see Japanese Patent Application Numbers Hei 7-208087 (208,087/1995) and Hei 7-208143 (208,143/1995)) that a coating that combines flexibility with high heat resistance is provided by the cure of a polymethylsilsesquioxane having a molecular weight and hydroxyl content in specific ranges and preferably prepared by a special method.
Japanese Patent Application Laid Open (Kokai or Unexamined) Number Sho 51-2736 (2,736/1976) discloses the dispersion of waterborne colloidal silica in a water-lower aliphatic alcohol solution of the partial condensate of RSi(OH).sub.3. However, this cannot be applied to steel sheet, etc., due to its acidity (pH=3 to 6). Moreover, the water repellency of the corresponding cured film is not high enough, as will be shown below in a comparative example.
Japanese Patent Publication (Kokoku) Number Sho 62-55554 (55,554/1987) discloses a waterborne coating composition with a pH of 7.1 to 7.8. This is a dispersion of waterborne colloidal silica in a water-aliphatic alcohol solution of the partial condensate of RSi(OH).sub.3. However, this reference makes no mention of the water repellency.
A coating originating from a composition containing organotrialkoxysilane and acidic colloidal silica (waterborne) is disclosed in Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 5-163463 (163,463/1993). Other essential components in this composition are additional organotrialkoxysilane, alcohol, and pigment. While this coating is reported to have an excellent hardness, water resistance, resistance to staining, aging resistance, and so forth, its water repellency (contact angle versus water) and processability (flexibility) are not elucidated and of course it is not transparent since it contains pigment.
Highly water-repellent coatings can be obtained by providing the coating surface with a microfine irregularity or roughness through the addition of relatively large particles with diameters in excess of 1 .mu.m, as disclosed in, for example, Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 3-244679 (244,679/1991). However, due to the presence of large particles whose size exceeds the thickness of the film itself, the physical properties of the film, such as hardness and processability, are poor, and of course the film is again not transparent.
Fluorocarbon resins do provide transparent and water-repellent films, but these films generally have a low surface hardness and thus do not always have a good staining resistance. These resins are also expensive.